[Driver][Linux] Remove D.Dir+"/../lib" from default search paths for LLVM_ENABLE_RUNTIMES builds

[MaskRay]

Summary

The rule was added in 2014 to support -stdlib=libc++ and -lc++ without
specifying -L, when D.Dir is not a well-known system library directory like
/usr/lib /usr/lib64. This rule turns out to get in the way with (-m32 for
64-bit clang) or (-m64 for 32-bit clang) for Gentoo :
https://github.com/llvm/llvm-project/issues/54515

Nowadays LLVM_ENABLE_RUNTIMES is the only recommended way building libc++ and
LLVM_ENABLE_PROJECTS=libc++ is deprecated. LLVM_ENABLE_RUNTIMES builds libc++
in D.Dir+"/../lib/${triple}/". The rule is unneeded. Also reverts D108286.

Gentoo uses a modified LLVM_ENABLE_RUNTIMES that installs libc++.so in
well-known paths like /usr/lib64 and /usr/lib which are already covered by
nearby search paths.

Implication: if a downstream package needs something like -lLLVM-15git and uses
libLLVM-15git.so not in a well-known path, it needs to supply -L
D.Dir+"/../lib" explicitly (e.g. via LLVMConfig.cmake), instead of relying on
the previous default search path.

[mgorny]

clang/lib/Driver/ToolChains/Linux.cpp

316

I wonder if this still wouldn't get in the way of some (non-Gentoo) people who have clang in /usr/bin/clang and 32-bit libc++ in /usr/lib/libc++.so.

[MaskRay]

clang/lib/Driver/ToolChains/Linux.cpp

316

It won't. /lib and /usr/lib follow this search directory. Since /lib and /usr/lib are usually the same, or at least include different sets of files, the user will not observe different behaviors.

[mgorny]

Thanks. Technically, it looks good to me. However, I'd suggest waiting until Jannik is able to test it.

[jdoerfert]

So, this broke all OpenMP (offloading) as libomp and (libomptarget) are no longer found by default. (OpenMP is build as ENABLE_RUNTIME.)

How do we fix this?

[jhuber6]

This change broke finding the OpenMP runtime libraries when using the LLVM runtimes build.

clang foo.c -fopenmp -fopenmp-targets=nvptx64
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: cannot find -lomp
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: cannot find -lomptarget

clang/lib/Driver/ToolChains/Linux.cpp

#endif
   if (IsRISCV) {
     StringRef ABIName = tools::riscv::getRISCVABI(Args, Triple);
     addPathIfExists(D, SysRoot + "/" + OSLibDir + "/" + ABIName, Paths);
     addPathIfExists(D, SysRoot + "/usr/" + OSLibDir + "/" + ABIName, Paths);
   }
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
-  // Similar to the logic for GCC above, if we are currently running Clang
-  // inside of the requested system root, add its parent library path to those
-  // searched.
-  // FIXME: It's not clear whether we should use the driver's installed
-  // directory ('Dir' below) or the ResourceDir.
-  if (StringRef(D.Dir).startswith(SysRoot)) {
-    // Even if OSLibDir != "lib", this is needed for Clang in the build
-    // directory (not installed) to find libc++.
+  // The deprecated -DLLVM_ENABLE_PROJECTS=libcxx configuration installs
+  // libc++.so in D.Dir+"/../lib/". Detect this path.
+  // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
+  if (StringRef(D.Dir).startswith(SysRoot) &&
+      D.getVFS().exists(D.Dir + "/../lib/libc++.so"))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
-    if (OSLibDir != "lib")
-      addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
-  }
 
   if (!D.OverlayToolChainPath.empty()) {
     addPathIfExists(D, ExtraPath + "/lib", Paths);
     addPathIfExists(D, ExtraPath + "/usr/lib", Paths);
   }
 
   addPathIfExists(D, SysRoot + "/lib", Paths);
   addPathIfExists(D, SysRoot + "/usr/lib", Paths);

clang/test/Driver/linux-ld.c

 // RUN:     -resource-dir=%S/Inputs/resource_dir \
 // RUN:     --sysroot=%S/Inputs/basic_linux_libcxx_tree \
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXX-INSTALL %s
 // CHECK-BASIC-LIBCXX-INSTALL: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-isysroot" "[[SYSROOT:[^"]+]]"
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-BASIC-LIBCXX-INSTALL: "--sysroot=[[SYSROOT]]"
-// CHECK-BASIC-LIBCXX-INSTALL: "-L[[SYSROOT]]/usr/bin/../lib"
 //
 // Test that we can use -stdlib=libc++ in a build system even when it
 // occasionally links C code instead of C++ code.
 // RUN: %clang -no-canonical-prefixes -x c %s -no-pie -### -o %t.o 2>&1 \
 // RUN:     -target x86_64-unknown-linux-gnu \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_linux_libcxx_tree/usr/bin \
 // RUN:     --gcc-toolchain="" \
 // RUN:     --sysroot=%S/Inputs/basic_linux_libcxx_tree \
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXX-C-LINK %s
 // CHECK-BASIC-LIBCXX-C-LINK-NOT: warning:
 // CHECK-BASIC-LIBCXX-C-LINK: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXX-C-LINK: "-isysroot" "[[SYSROOT:[^"]+]]"
 // CHECK-BASIC-LIBCXX-C-LINK-NOT: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v1"
 // CHECK-BASIC-LIBCXX-C-LINK: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-BASIC-LIBCXX-C-LINK: "--sysroot=[[SYSROOT]]"
-// CHECK-BASIC-LIBCXX-C-LINK: "-L[[SYSROOT]]/usr/bin/../lib"
 //
 // Check multi arch support on Ubuntu 12.04 LTS.
 // RUN: %clang -no-canonical-prefixes %s -no-pie -### -o %t.o 2>&1 \
 // RUN:     --target=arm-unknown-linux-gnueabihf -rtlib=platform \
 // RUN:     --gcc-toolchain="" \
 // RUN:     --sysroot=%S/Inputs/ubuntu_12.04_LTS_multiarch_tree \
 // RUN:   | FileCheck --check-prefix=CHECK-UBUNTU-12-04-ARM-HF %s
 // CHECK-UBUNTU-12-04-ARM-HF: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"